As different wireless analog and digital cellular telephone systems and satellite systems are promulgated throughout the world, antennas corresponding to each of the systems are developed. System subscribers who travel through different systems or who use a telephone in a geographical area with more than one system, desire a single telephone usable on more than one system. Communication on differing bands of frequencies in the same radio is therefore desired. Because antennas of different bands for the same telephone could likely be inconvenient for a user, a single antenna structure capable of operation at more than one band is desired.
In a radio frequency (RF) communication system, a user of a handheld subscriber unit (SU) on a call can experience fading of the call due to multipath. Multipath is a phenomena by which out of path signals, such as out of phase noise signals, add to the main signal to produce a distorted signal in the SU. This multipath signal arrives at the SU and causes a fade to occur when the multipath signal is combined with the main path signal. During this fade, it will appear to one or more parties on the call that the phone call has been dropped.
Fading due to multipath typically results in power losses of 10-40 dB, requiring other components along the path from the SU's transmitter to receiver (link budget) to compensate for severe fades in order to preserve the quality of service. This tends to be especially true for a hand-held mobile unit such as the IRIDIUM.RTM. satellite SU and other satellite and terrestrial mobile telephony systems. Fades can last a rather long period of time, such as several tens of seconds for a slow moving, terrestrial based person who is walking along at a normal pace.
Research into fading has demonstrated that in a mobile RF communication system with an antenna having dual, orthogonal polarizations, multipath fading affects one polarization of the antenna for a period of time and then begins to affect the other polarization as the orthogonal polarization recovers.
U.S. Pat. No. 4,554,554 ('554 patent) discloses a quadrifilar helix antenna whereby PIN diodes are placed at predetermined locations on the antenna coaxial cable radiating elements for tuning the antenna in separate discrete frequency bands. However, the quadrifilar antenna of the '554 patent is not a dual orthogonally polarized antenna, and the '554 patent does not address the effects of multipath on a dual orthogonally polarized antenna.
PCT published application No. PCT/US96/10459 discloses a double helix antenna system including a first helix conductor wound in a first direction about a vertical axis of the double helix antenna and a second helix conductor wound in a second direction about the longitudinal axis of the antenna. In this system, the two conductors have to be physically orthogonal to each other at the points of intersection in order to provide minimal coupling and increase electrical isolation of one conductor from the other. The orthogonal winding relationship enables operation of separate helical antennas in close physical proximity. However, the orthogonality of the conductors at the point of intersection necessitates a given pitch for each conductor and may limit the frequency range of the antenna and its utility to a roaming user.
A need therefore exists for a multi-polarized, multiple band antenna arrangement that can decrease the effect of fading of a call due to multipath without compromising the frequency range and utility of the antenna.